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       icmp, IPPROTO_ICMP - Linux IPv4 ICMP kernel module.


       This  kernel  protocol  module  implements the Internet Control Message
       Protocol defined in RFC 792.  It is used to signal error conditions and
       for  diagnosis.   The  user doesn’t interact directly with this module;
       instead it communicates with the other  protocols  in  the  kernel  and
       these  pass the ICMP errors to the application layers.  The kernel ICMP
       module also answers ICMP requests.

       A user protocol may receive ICMP  packets  for  all  local  sockets  by
       opening  a  raw  socket with the protocol IPPROTO_ICMP.  See raw(7) for
       more information.  The types of ICMP packets passed to the  socket  can
       be  filtered  using  the  ICMP_FILTER  socket option.  ICMP packets are
       always processed by the kernel too, even when passed to a user  socket.

       Linux  limits  the  rate  of  ICMP  error  packets to each destination.
       ICMP_REDIRECT and ICMP_DEST_UNREACH are also limited by the destination
       route of the incoming packets.

       ICMP   supports   a  sysctl  interface  to  configure  some  global  IP
       parameters.  The sysctls can be accessed  by  reading  or  writing  the
       /proc/sys/net/ipv4/*  files  or  with the sysctl(2) interface.  Most of
       these sysctls are rate limitations for specific ICMP types.  Linux  2.2
       uses a token bucket filter to limit ICMPs.  The value is the timeout in
       jiffies until the token bucket filter is  cleared  after  a  burst.   A
       jiffy is a system dependent unit, usually 10ms on i386 and about 1ms on
       alpha and ia64.

              Maximum rate to send ICMP Destination Unreachable packets.  This
              limits  the  rate  at  which  packets are sent to any individual
              route or destination.  The limit  does  not  affect  sending  of
              ICMP_FRAG_NEEDED packets needed for path MTU discovery.

              If  this  value  is  non-zero,  Linux  will ignore all ICMP_ECHO

              If this value is  non-zero,  Linux  will  ignore  all  ICMP_ECHO
              packets sent to broadcast addresses.

              Maximum  rate  for sending ICMP_ECHOREPLY packets in response to
              ICMP_ECHOREQUEST packets.

              Maximum rate  for  sending  ICMP_PARAMETERPROB  packets.   These
              packets  are  sent  when  a  packet  arrives  with an invalid IP

              Maximum rate  for  sending  ICMP_TIME_EXCEEDED  packets.   These
              packets  are sent to prevent loops when a packet has crossed too
              many hops.


       Support for the ICMP_ADDRESS request was removed in 2.2.

       Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.


       As many other implementations don’t support IPPROTO_ICMP  raw  sockets,
       this feature should not be relied on in portable programs.

       ICMP_REDIRECT  packets  are  not  sent  when  Linux  is not acting as a
       router.  They are also only accepted from the old  gateway  defined  in
       the  routing table and the redirect routes are expired after some time.

       The 64-bit timestamp returned  by  ICMP_TIMESTAMP  is  in  milliseconds
       since January 1, 1970.

       Linux ICMP internally uses a raw socket to send ICMPs.  This raw socket
       may appear in netstat(8) output with a zero inode.



       RFC 792 for a description of the ICMP protocol.


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